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Hi! I’m working on a project in which I have to sense the output of a sensor that varies between +50V and -50V. I’m only interested in values between +5V and -5V, values higher than 5 V and lower than -5V must be discharged.
My ADC supports inputs between 0V and 3,3V. So to cover all the values between +5 and -5 I’ve implemented a signal conditioner whit INA128 and OPA177 in Offset trimming configuration with the following characteristics:
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- Offset voltage: +1,65V.
- R1=2,03K
- R2=1K
- Vin is attenuated 33% with resistor divider formed by R1 and R2.
- Both INA128 and OPA177 are powered with +5V and -5V supplies.
- Rg=Not connected ; Gain=1.
This circuit is already tested and working perfectly always and when then voltage input is between +5V and -5V.
Examples:
·
For
Vin=5V ; Vout=5*0,33 + 1,65= 3,3V (max ADC input)
·
For
Vin=-5V ; Vout=-5*3,3 + 1,65 = 0V (min ADC input)
Normal 0 21 false false false ES X-NONE X-NONE MicrosoftInternetExplorer4 The ADC “Limiting values” for input is: Vin_max=5V ; Vin_min= -0,5V. (Note that these ones are the maximum and minimum values allowed that do not damage the ADC, but to work ok the ADC needs values between 0V and 3,3V)
The problem appears if the input is lower than -5V, say for example -10V. Vout=-10*0,33 + 1,65 =-1,65V. This value can damage my ADC. So the question is, is there any way to avoid INA128 output going lower than 0V, or at least not lower than -0,5V?
Whit values higher than 5 V there are no problems because INA128 saturates at 4V aprox avoiding ADC damage. Obviously ADC does not work ok with 4V but at least doesn’t suffer any damage.
Thanks in advance, Marcos
